BOURG-SAINT-ANDEOL, France (Reuters) - The Tour de France resumed under heightened security and in a low-key mood on Friday after race officials observed a minute's silence in tribute to the scores of victims of the Bastille Day attack in Nice.

A gunman at the wheel of a heavy truck ploughed into a crowd celebrating France's national day in the southern city late on Thursday, killing at least 84 people and injuring scores in what President Francois Hollande called a terrorist act.

After race organisers debated whether it should be cancelled, the 13th stage of the signature event of the country's sporting summer began at 0805 GMT, with its publicity caravan - usually playing loud music to energise the thousands lining the route -- silenced for the day.

"We had a crisis meeting with the prefect of the Ardeche department (hosting the stage) and the gendarmerie. The stage is on," said race director Christian Prudhomme.

"We want this day to be a day of dignity as a tribute to the victims... We think, after agreeing with authorities, that the race must continue."

That decision was taken "in direct liaison with the highest authorities of the state" Lieutenant-Colonel Eric Luzet, the gendarmerie's liaison officer to the tour, told Reuters.

"We have 600 officers mobilised to ensure security on this stage. They’re at the start, at the finish and along the route.. After what happened last night in Nice we have changed our security feature accordingly."

Special gendarmerie forces, also known as GIGN, have accompanied the three-week race since it started on July 2. The force now had an extra helicopter at its disposal in the event of a threat or attack, Luzet said.

The Tour ends on July 24 in Paris, on the Champs Elysees.

Friday's stage is a 37.5-km (23.5-mile) time trial from Bourg-saint-Andeol to Vallon Pont-d'Arc in the Ardeche region some three hours' drive northwest of Nice.

Race leader Chris Froome of Britain will be the last man on the road, and another minute's silence will be held at the finishing line, Prudhomme said.